1. Field of the Invention
The present invention relates to a technique for reducing color misregistration for use in an image forming apparatus.
2. Description of the Related Art
The speed at which an image is formed in recent color image forming apparatuses is becoming increasingly higher. With this, color misregistration (a problem in which images of different colors that should be formed in the same position are formed in different positions on a recording material) resulting from a plurality of factors may occur. A main factor is the accuracy with which an optical unit arranged for each color is mounted.
This problem is present especially in a tandem color image forming apparatus, which includes the same numbers of developing units and photosensitive drums as coloring materials and sequentially transfers images for different colors onto an intermediate transfer member or a recording medium.
FIGS. 19A and 19B are illustrations for describing an example of color misregistration in a main scanning direction. An image position 201 is an image position at which an image should be formed, and image positions 202a and 202b are positions of an image suffering color misregistration. In FIGS. 19A and 19B, for the sake of illustration, the gap between two lines extends in the direction of conveyance. FIG. 19A illustrates an error of a position at which image formation starts in the direction in which sheets are conveyed. This can be corrected in the direction of the arrow by, for example, adjustment of the time of starting image formation for each color from detection of the leading end of the sheet. FIG. 19B illustrates an error of a position at which image formation starts in the main scanning direction. When a laser scanner is used as an optical unit, this can be corrected in the direction of the arrow by, for example, adjustment of the time of starting image formation from a position where a beam is detected.
There is a technique for correcting such color misregistration by generating a color misregistration detection pattern for each color on an intermediate transfer member, detecting the pattern by using an optical sensor disposed downstream of the intermediate transfer member, determining the amount of color misregistration, and correcting the color misregistration (see, for example, Japanese Patent Laid-Open No. 10-260567).
However, the known technique cannot be applied to an apparatus that is operable in modes for different scanning speeds and that can perform scanning for a reference color (first color) and scanning for another color (second color) in different scanning directions, so the color misregistration problem still remains.